A single radio voice call continuity (SRVCC) function is used for voice over long term evolution (VoLTE) providing voice service on long term evolution (LTE), the SRVCC function making a terminal make a transition to a non-LTE network to continue voice service when the radio quality of an LTE network is degraded. VoLTE is a technology for providing voice communication as packet communication.
The following technology is known as another related technology. An evolved node B (eNB) includes a handover control unit. The handover control unit sets an LTE system, a voice over internet protocol (VoIP) bearer, and another communication bearer that is not used for the VoIP, and changes the coupling destination of a coupled mobile apparatus to a 3rd generation (3G) system. At this time, the handover control unit hands over only the VoIP replaced with a circuit switched (CS) bearer in the 3G system by SRVCC on the basis of priority of the VoIP bearer over the other bearer that is not used for the VoIP. Then, the handover control unit does not hand over the other bearer that is not used for the VoIP.
In addition, the following technology is known as another related technology. A handover of a user terminal between a packet switched domain and a circuit switched domain is indicated. A single radio voice call continuity activation event is detected. Control plane signaling radio bearers are suspended so as to be consistent with a serving radio network subsystem relocation procedure. The suspended signaling radio bearers are reset. The suspended signaling radio bearers in the domain to which the handover is performed are resumed. The resuming includes protecting the control plane signaling radio bearers of the domain to which the handover is performed, using a same mapped security key that is used to cipher user plane radio access bearers in the domain to which the handover is performed. Prior art documents include Japanese Laid-open Patent Publication No. 2012-186855 and Japanese Laid-open Patent Publication No. 2013-532404.